1. Field of the Invention
The invention relates to the field of photodetector devices and methods of photodetection, and in particular to photodetector arrays used in combination with optical waveguides.
2. Description of the Prior Art
Ultrafast photodetectors with high quantum efficiencies and high saturation power are required for microwave fiber-optic links and for optoelectronic generation of microwaves and millimeter waves. With the development of the erbium doped fiber amplifiers, ultrafast high-power photodetectors are also becoming necessary in high speed optical communication systems. In analog and microwave applications, high optical power is desired to improve the link gain in signal-to-noise ratio. However, ultrafast photodetectors tend to saturate at very low optical power because of the small detector sizes and high optical power density. Therefore, what is needed is a ultrafast photodetector with a high optical saturation power.
The performance of ultrafast photodetectors are measured by the product of bandwidth and efficiency. Conventional photodetectors with surface-normal illumination have a bandwidth-efficiency product of approximately 30 GHz. In conventional photodetectors, light propagates in the direction parallel to the transport of the electrical carriers and in order to achieve high efficiencies, a thick absorbing layer is required. The bandwidth of such devices is therefore limited by the long carrier transport time. As described by J. E. Bowers et al., "Ultrawide-Band Long-Wavelength P-I-N Photodetectors," IEEE J. Lightwave Technology, Volume LT-5, Number 10, Pages 1339-50 (October 1987) the product of the bandwidth and efficiency is limited to 0.45 av (1-R) where a is the absorption coefficient, v is the carrier velocity and R the reflectivity. Using typical device parameters, a conventional 100 GHz photodetector will then have an efficiency typically no higher than 31 percent.
Several approaches have been proposed in the prior art to increase the bandwidth-efficiency product of photodetectors, including cavity resonance-enhanced photodetectors (surface illumination) K. Kishino et al., "Resonant Cavity Enhanced (RCE) Photodetectors," IEEE J. Quantum Electron. Vol. QE-27, PP 2035-43 (1991) and A. Chin et al., "Enhancement of Quantum Efficiency in Thin Photodiodes Through Absorptive Resonance," J. Lightwave Technology, Volume 9, PP 321-28 (March 1991) and waveguide photodetectors (edge Illumination), J. E. Bowers et al., "Ultrawide-Band Long Wavelength P-I-N Photodetectors", IEEE J. Lightwave Technology, Volume LT-5, Number 10, PP 1339-50 (October 1987).
The prior art has attempted to avoid the fundamental limit imposed by the long carrier transport time by using waveguide photodetectors in which light propagates perpendicular to the carrier transport direction. However, these devices then encounter another upper limit, namely the "walk off" between the inputted light and the detected microwave signal along the waveguide due their velocity mismatch. A traveling wave photodetector has been proposed to increase the bandwidth-efficiency product, K. S. Giboney et al., "Traveling Wave Photodetectors," IEEE Photonics Technology Letters, Volume 4, Pages 1363-65 (1992). The bandwidth is, however, limited by velocity mismatch between the optical waves and the microwaves.
Very high bandwidth metal-semiconductor-metal (MSM) photodetectors using submicron finger width have also been devised as an attempted solution to this problem. Y. Chen et al., "375 GHz Photodiode on Low Temperature Gallium Arsenide," Appl. Phys. Lett. Volume 59, Pages 1984-86 (1991) and S. Y. Chou et al., "Nanoscale Terahertz Metal-Semiconductor-Metal Photodetectors," IEEE J. Quantum Electron, Volume 28 Number 10, PP 2358-68 (October 1992).
However, each of these approaches fails to address the limitation of high power saturation and, in fact, the resonance approach actually reduces the saturation power limit. Therefore, what is needed is a new photodetector design which has a high bandwidth and high efficiency in which the velocity matched structure still allows optical saturation power to be greatly increased.